If you gaze out into the night sky with a telescope, and see beyond what’s visible to the naked eye, you could see a lot of “stars” that are actually imposters. Many of the points of light we often think are individual stellar objects are actually galaxies, collections of millions to trillions of stars. Galaxies are composed of stars, dust, and dark matter, all held together by gravity. Below we discuss galaxy formation, galactic collisions and other facts about these so-called “island universes.”
Galactic & Black Holes
Galaxies come in a variety of shapes, sizes, and ages. Many have black holes at their centers. In some cases, a galaxy’s central black hole is extremely large or active, it’s surrounding area producing a tremendous amount of energy that astronomers can see over great distances. Material circling the black hole may be accelerated outward by its jets. Other galaxies may contain objects like quasars, the most energetic bodies in the universe, at their core.
Astronomers aren't certain of exactly how galaxies formed. After the Big Bang, space was made up almost entirely of hydrogen and helium. Some astronomers think that gravity pulled dust and gas together to form individual stars, and those stars drew closer together into collections that ultimately became galaxies. Others think that the mass of what would become galaxies drew together before the stars within them were created.
In the 1900s, many astronomers thought that the entire universe lay within our galaxy, the Milky Way. Others argued that the spiral-shaped blobs thought to be dust and gas were separate; Harlow Shapley called them "island universes." It wasn't until 1924, when Edwin Hubble identified several special pulsing stars called Cepheid variables in some of these so-called nebulae and realized that they lay outside of the known span of the Milky Way, that astronomers realized they were, in fact, completely unique collections of stars at distances well beyond our home galaxy.
After Hubble measured the distance to individual galaxies, he went on to measure their Doppler shift — how much light from the galaxies was stretched out due to their motion. He determined that galaxies all around the Milky Way are moving away from us at terrific speeds. The farther away the galaxies are, the faster they are fleeing. Because of this, he was able to determine that the universe itself is expanding. Later astronomers determined that this expansion is accelerating.
Galaxies are classified by their shape. Each type has different characteristics and a different history of evolution.
Some, like the Milky Way, have arms spiraling outward around their center. Known as “spiral galaxies,” these groups make up most of the galaxies that astronomers can see. The gas and dust in a spiral galaxy circles the center at speeds of hundreds of miles per second, creating their pinwheel shape. Some, known more precisely as “barred spirals,” have a bar structure in their center, formed by dust and gas funneled into the center. Present in all spirals, the dust and gas fuel star formation, so spiral galaxies are constantly forming stars today.
Elliptical galaxies lack the spiral arms of their more flamboyant cousins. Their appearance ranges from extremely circular to very stretched out. Elliptical galaxies have less dust than their spiral counterparts, and so the star-making process has all but ended. Most of their stars are older. Although they make up a smaller portion of the visible galaxies, astronomers think that over half the galaxies in the universe are elliptical.
The remaining 3 percent of the galaxies in the universe are known as irregular galaxies. They are neither round nor boast spiral arms, and their shapes lack specific definition. The gravity of other galaxies has often affected them, stretching them out or warping them. Collisions or close calls with other galaxies can also deform their shapes.
When galaxies collide
Galaxies don't float through space in isolation. They are bunched together in groups known as clusters. Some clusters are large, containing over a thousand galaxies. Others are smaller. The Milky Way lays within the cluster known as the Local Group, which only contains 50 galaxies.
Occasionally, they slam into one another, merging their stars and dust together. This is an important step in the evolution and growth of many galaxies.
Individual stars generally don't collide in a galactic collision, but the influx of dust and gas bumps up the rate of star formation. The Milky Way is set to collide with the Andromeda galaxy in about 5 billion years.
— Nola Taylor Redd